1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus for forming a multicolor image by electrophotography.
2. Description of the Prior Art
A number of proposals have been made in the art as to the multicolor image forming apparatus using the electrophotography. The apparatus can be generally classified into the following broad categories. In a first category, electrostatic latent images having their colors separated are repeatedly formed and developed on a single photosensitive member so that the colors are superposed on the photosensitive member, or the toner images are transferred to a transfer material in each development so that the colors are superposed on the transfer material. In a second category, toner images in different colors are simultaneously formed on respective photosensitive members, the number of which corresponds to the number of colors, and are sequentially transferred to a transfer material to obtain a multicolor image. The latter apparatus is advantageous in its high speed because the formation of the toner images in respective colors is simultaneously conducted on the photosensitive members, respectively. Since it requires plural photosensitive members and exposing means, however, the apparatus is complicated and large-sized to raise its production cost so that its practicability is not good enough. Moreover, both of the above-specified multicolor image forming apparatus are seriously defective in that the registration is difficult in color superposition so that the color drift of the image cannot be completely prevented.
In order to drastically solve these problems, I have invented an apparatus for forming a multi-color image through a single image exposure on a photosensitive member. In this apparatus the multi-color image formation is conducted in the following manner by using a photosensitive member which is composed of an electroconductive member, a photoconductive layer and a plurality of different kinds of filters. By charging and image-exposing the surface of the above-specified photosensitive member, more specifically, an image of charge density is formed on the boundary surface between the insulating layer and the photoconductive layer. The boundary surface on which the image has been thus formed is wholly exposed to specified light to form a potential pattern in said filter portion of the aforementioned photosensitive member. This potential pattern is developed to form a monocolor toner image by a developing device which contains toner in a specified color. Then another whole surface exposure using light through a filter portion different from the preceding one and another development using a developing device containing toner in a color different from the preceding one are conducted to form a toner image in a second color on the photosensitive member. Subsequently, a desired number of whole surface exposures and developments are repeated. As a result, toners in the different colors stick to the respective filter portions of the photosensitive member of form a multicolor image (as should be referred to Japanese Patent Application Nos. 59-83096 and 59-187044). According to the multicolor image forming apparatus thus disclosed, there arises no fear of color drift because of the single image exposure.
My invention described above has succeeded in eliminating the aforementioned problems accompanying the multicolor image forming apparatus of the prior art. However, my further investigations have revealed that the following problems are still left unsolved.
The multicolor image forming apparatus described effects the color reproduction by the so-called "color adding method", in which the colors are not superposed in principle in the same position. For reproduction of black by toners of three colors of yellow, magenta and cyan, for example, these toners are so arranged on a recording member that they are not superposed on others, and the black is expressed as a composite of reflected light of the respective color components. However, this method has a tendency to make the monocolor image short of density. This is because the optical reflections of the respective toners on the recording member are of high intensity. For example, the black reproduced appears grey to the viewer even if the color balance is complete.
Moreover, it is difficult for the aforementioned filters to have ideal spectral characteristics, and most of them permit light other than visible light, especially infrared rays, to pass therethrough. On the other hand, many photosensitive members are sensitive not only to visible light but also to infrared rays or ultraviolet rays. Despite of these facts, the design of the spectral distribution of the filters and photosensitive members cannot be freely changed so that infrared rays or ultraviolet rays disturb the distribution of the sticking toners in color reproduction to fail in conducing the color reproduction in a high fidelity. Some reasons for this failure will be described in the following.
The filters each has a high transmissivity in the infrared range as shown in FIG. 21. On the other hand, a photosensitive member is required to have a panchromatic sensitivity over the visible range, 400 to 700 nm, in wavelength. Many photoconductive layers (which are made of Se (Te, Sb or As), As.sub.2 Se.sub.3, Cu phthalocyan, a(amorphous)-Si or the like) for such a photosensitive member have a sensitivity not only in the infrared range (longer than 700 nm in wavelength) but also in the ultraviolet range (shorter than 400 nm). Such a combination of filters and photosensitive members is not special but rather general.
If such a photosensitive member undergoes an image exposure using light containing infrared and or ultraviolet rays, color information in the infrared and ultraviolet ranges is stored in the photosensitive member underlying the filters so that the color reproduction is not carried out in a high fidlity.
In the case of an image exposure containing infrared rays, for example, the blue (B), green (G) and red (R) filters are transparent to infrared rays so that an infrared image (in a color corresponding to infrared rays of 700 nm or more, especially 750 nm or more) transmits through all the filters to form a primary latent image in the photosensitive member. If this photosensitive member is subjected to a whole surface exposure to light in a specified color to form a secondary latent image and is then developed using yellow, magenta and cyan toners, the color reproduction is conducted not in a high fidelity such that the aforementioned infrared image portion is superposed on a visible image (within the wavelength range of 400 to 700 nm).
A similar problem arises, too, in the conventional multicolor image forming apparatus in which the image exposure, development and transfer are repeated for each color component, as described hereinbefore. In such a multicolor image forming apparatus of the prior art, however, the problem can be avoided by ignoring the resultant increase in the production cost by disposing a filter having excellent spectral characteristics such as an interference filter in the optical path. Despite of this possibility, however, it is impossible in the present technical state for the multicolor image forming apparatus, which has its photosensitive member formed of layers of several kinds of color separation filters, to be finely arranged with filters having as excellent spectral characteristics as an interference one.